The present invention relates to control systems for aircraft, aerospace vehicle or the like and more particularly to a bias correcting phase compensating rate limiter and method.
Mechanical devices, such as actuators or the like, are typically used in fixed wing aircraft, helicopters, and tilt rotor aircraft to control movement of flight control members or surfaces, such as elevators, rudders, ailerons, engines, rotors, swashplates, blades and the like. These actuators are generally designed so that their maximum possible rate of displacement is low enough that actuator failures or inadvertent large and rapid pilot stick inputs cannot cause aircraft structural failure or loss of aircraft control. When it is impractical or inefficient to design actuators so that their maximum rate of motion is mechanically or hydraulically limited, the electronic circuitry or digital software that generates the driving input signals to the actuator is often designed so that the actuator is never commanded to move beyond a specified rate limit. In many instances, rapid maneuvering or quick operation of flight controls, such as rudder pedals, center stick, collective lever or other such controls can saturate or exceed the physical rate limit capabilities of these actuators or mechanisms or exceed the equivalent electronic rate limits implemented in the analog circuitry or software that commands the actuators. Actuator or software rate limit impingement can result in a phase lag causing operation of the actuator to lag behind movements of the controls by the pilot. This phase lag in actuator response significantly delays aircraft response to rapid pilot inputs, making it difficult for pilots to predict the effect of their stick inputs and control the aircraft with the precision necessary for aviation. FIG. 1 is a graph 100 of an input signal 102 compared to an output signal 104 of a prior art rate limiter that is not bias corrected or phase compensated. As illustrated, without phase compensation, the output signal 104 will lag behind the input signal 102.
Prior art methods attempt to compensate for the phase lag in actuator response that occurs when actuator rate limits are saturated. However, the prior art methods for rate limiter phase compensation produce a significant undesirable side effect. When a rate limiter is adapted to compensate for phase lag using prior art methods, biases or sustained differences in the magnitude between an input signal to the rate limiter and the output signal controlling the mechanical device or actuator can result. FIG. 2 is a graph 200 of an input signal 202 compared to an output signal 204 of a prior art phase compensated rate limiter that is not bias corrected. As illustrated in FIG. 2, short term perturbations in the output signal 204 may be substantially in phase with short term perturbations in the input signal 202 but there is a distinct bias between the long term moving averages of the input signal 202 and the output signal 204. The long term biases introduced by prior art phase compensating rate limiters can result in aircraft control response predictability that is at least as poor as that experienced without phase compensation and in worst case scenarios could result in a complete loss of aircraft control if the bias grows too large.